1. Field of the Invention
The invention relates to a method of manufacturing a device such as a semiconductor laser device. The invention also relates to a crystal substrate of a III-V compound of the nitride system and a crystal film of a III-V compound of the nitride system, which are used for the method of manufacturing a device. The invention also relates to a method of manufacturing a crystal of a III-V compound of the nitride system, which is used for manufacturing the crystal substrate or the crystal film.
2. Description of the Related Art
In general, the manufacturing process of a device such as a semiconductor laser device or a light-emitting diode (LED) involves the deposition of semiconductor films or the like laid one upon another over the surface of a crystal substrate or a crystal film comprised of a III-V compound of the nitride system, e.g., gallium nitride (GaN). A desirable method to obtain a crystal substrate or a crystal film for the use in the manufacturing process is to form a bulk of a crystal of a III-V compound of the nitride system, but the manufacture of a bulk of a crystal of a III-V compound of the nitride system is difficult. Thus, it has been the practice to grow epitaxially a crystal of a III-V compound of the nitride system on a basal body made of sapphire (Al2O3).
However, the difference in the crystal structures of the basal body and the crystal formed thereon, or the small interaction between the basal body and the crystal formed thereon causes dislocations to occur from the interface. Such dislocations develop in the direction of crystal growth, piercing the crystal to reach its surface. This causes a problem that crystal substrates or crystal films thus grown have numerous defects in the surface.
To overcome the problem, Publication of Japanese Unexamined Patent Application No. Hei 10-312971 proposes a method in which a gallium nitride (GaN) base layer is formed over the surface of a sapphire (Al2O3) basal body, and a mask pattern made of silicon dioxide (SiO2) is formed over the base layer, and then a gallium nitride (GaN) crystal is grown over the base layer with the mask pattern in between. According to the method proposed, the development of dislocations is prevented by the mask pattern, resulting in a reduction in the number of dislocations piercing the crystal to reach its surface. The method, however, cannot sufficiently reduce the number of dislocations in crystal substrates or crystal films because some dislocations develop through windows of the mask pattern to pierce the crystal.
Thus, the above-noted Publication discloses another method in which two mask patterns are formed in the direction of the thickness of the crystal so that a dislocation developing through a window of one mask pattern is blocked by another mask pattern. This method, however, requires accurate alignment of two mask patterns in the direction of the thickness of the crystal so that one mask pattern is laid over the windows of another mask pattern, which causes a problem of difficult work.
According to Extended Abstracts (The 46th Spring Meeting, 1999); The Japan Society of Applied Physics and Related Societies, page 416, disclosed is a method in which a base layer made of gallium nitride (GaN) or the like is formed over a sapphire (Al2O3) basal body, and a recessed region is formed in the surface of the base layer, and a gallium nitride (GaN) crystal is grown over the surface of the base layer. Since the method causes a change in the direction of the development of dislocations at the recessed region of the base layer, the number of dislocations piercing the crystal reduces to some extent. In the method, however, dislocations propagate through the portion other than the recessed region of the base layer. Thus, the method has a problem that the number of dislocations piercing the crystal cannot be reduced sufficiently.
On the other hand, according to MRS Internet J. Nitride Semicond. Res. 4S1, G3. 38 (1999) and MRS Internet J. Nitride Semicond. Res. 4S1, G4. 9 (1999), proposed are methods of preventing dislocations piercing the crystal using lateral growth. In the former method, a groove is formed in a gallium nitride (GaN) crystal as a seed crystal, and the crystal growth is restarted in the lateral direction from the walls of the groove.
In the latter method, after a mask pattern is formed on the surface of a gallium nitride (GaN) crystal as a seed crystal, the gallium nitride (GaN) crystal is etched to form a groove, and the crystal growth is re-started in the lateral direction from the walls of the groove, while the mask pattern prevents the crystal growth over the surface of the seed crystal. However, with the former method, there is a possibility that dislocations will develop from the surface of the seed crystal. With the latter method, there is a possibility that other dislocations will develop above the mask pattern. Thus, both methods have a problem that defects in the surface of the crystal cannot be prevented sufficiently.